This invention relates to a variable focal length optical system which is focusable in a normal range and in a closeup range. More specifically, the present invention relates to a zoom lens system which is capable of macrophotography.
The typical zoom lens system has a variator lens and a compensator lens, each of which is disposed in camming engagement with a cam tube which is rotatable for mutually simultaneously effecting cammed axial movements of the variator and compensator lenses for focusing the system in a normal zoom range between a telephoto condition and a wide-angle condition. In order to make macrophotography possible in such a zoom lens system, provision is typically made at one and/or both ends of the normal zoom range for holding either the variator lens or the compensator lens stationary while moving only the other one for focusing in a macrophotographic range.
In one form of zoom lens system, this selective control of the variator and compensator lenses is accomplished by splitting the cam tube into two parts, respectively for controlling the variator lens and the compensator lens. When the system is operated in the normal zoom range, the two parts of the cam tube are rotated together, and when macrophotography is to be effected, the two cam tubes are de-coupled from each other so as to be operable independently of each other. In such a system, since the cam tube is separated into two portions, the moving range of each lens is limited to the length of the corresponding portion of the cam tube.
In other zoom lens systems, this problem is avoided by forming the macrophotography cam slots as continuous extensions of the normal zoom range cam slots in a unitary cam tube. Such an arrangement is disclosed in the copending application of Rudolph Hartmann, Ser. No. 744,835, filed Nov. 24, 1976, now abandoned and assigned to the assignee of the present invention. By such an arrangement, the moving range of each of the variator and compensator lenses can be spread wider than in the case of a split cam tube, but the increased length of the cam slots tends to weaken the cam tube.
Still another arrangement is disclosed in U.S. Pat. No. 3,891,310, wherein the cam tube is stationary, the cam pins for the variator lens and compensator lens extending completely through the corresponding cam slots in the stationary cam tube for engagement with a rotatable outer operating sleeve. The operating sleeve has a normal position wherein it engages the cam pins of both lenses for operation in the normal zoom range, and a retracted position wherein it engages only one of the lens cam pins for operation in a macrophotographic range. But this arrangement necessitates the use of an additional operating sleeve, and it also has the disadvantage of causing rotation of the variator and compensator lenses as they are moved axially by the action of the cam slots. Furthermore, since the cam slot used for controlling the movement of the variator lens in the macrophotographic range is the same as that used for controlling that lens during the normal zone range, the configuration of that cam slot limits the movement which the variator lens can undergo in the macrophotographic range. This is a major limitation since, in most macrophotographic applications, it is desirable to move one or the other of the variator lens and the compensator lens in a manner different from its movement during the normal zoom range operation and, indeed, it is typically desirable that either the variator lens or the compensator lens be held stationary in the macrophotographic range.